Make shared library targets more consistent

On Windows POSIX layers, two files are produced for a shared library,
there's {shlibname}.dll and there's the import library {libname}.dll.a

On some/most Unix platforms, a {shlibname}.{sover}.so and a symlink
{shlibname}.so are produced.

For each of them, unix-Makefile.tmpl was entirely consistent on which
to have as a target when building a shared library or which to use as
dependency.

This change clears this up and makes it consistent, we use the
simplest form possible, {lib}.dll.a on Windows POSIX layers and
{shlibname}.so on Unix platforms.  No exception.

Reviewed-by: Rich Salz <rsalz@openssl.org>